This invention relates to an improved heated fluidized bed reactor, and a method for heating such reactor. In a preferred embodiment, the present invention relates to an improved heated fluidized bed reactor used in the production of polycrystalline silicon by the pyrolysis of silane containing gases.
A variety of means are known in the art for supplying the necessary heat to fluidized bed reaction zones. For example, in the pyrolysis of silane containing gas to silicon, heat can be provided by capacitive heating of the fluidized bed reaction zone, as discussed in U.S. Pat. No. 4,292,344 to McHale. Other methods of heating such as uniform induction coils, electrical resistance elements and indirect gas fired heaters have also been used and are disclosed in U.S. Pat. Nos. 3,012,861 to Ling and 3,012,862 to Bertrand et al. A suitable heat transfer fluid and inductive or electrical resistance heaters are also examples of means for directly supplying heat to the surfaces of conventional fluidized bed reactors. While adequate for the purposes of some fluidized bed applications, these means are not always satisfactory for other fluidized bed applications because of the nature of the reactions occurring therein and the heat requirements of the fluidized bed.
For instance, in the production of polycrystalline silicon from silane containing gases in a fluidized bed reaction zone, conventional means for supplying heat to the wall of the fluidized bed are generally unsatisfactory. In this example, silicon particles are suspended in a fluidizing gas stream into which silane containing gases are introduced. The process conditions are desirably maintained so that the decomposition of the silane containing gas occurs heterogeneously on the surface of the silicon particles of the fluidized bed, rather than on the hot wall of the fluidized bed. The silicon particles grow and enlarge by the deposit of silicon thereon so that sufficiently large silicon product particles are produced and removed from a collection zone below the reaction zone.
Supplying heat to the wall of the fluidized bed reaction zone causes the temperature of the wall to be higher compared to the temperature of the silicon particles. This may result in an undesired deposition of the silicon on the wall of the reaction zone, in preference to the desired deposition of the silicon onto the surface of the fluidized silicon particles. In addition to reducing the amount of silicon that deposits onto the surface of the silicon particles, the deposition of silicon on the reactor walls has the effect of reducing the heat transfer efficiency into the reaction zone because of the additional layer through which the heat must travel.
Therefore, a need exists for an improved heated fluidized bed reactor, useful, for example, as an improved heated fluidized bed reactor for the production of high purity polycrystalline silicon.